This invention relates to the general field of compliant foil fluid film bearings.
Compliant foil fluid film thrust bearings are currently being utilized in a variety of high speed rotor applications. These rotor bearing systems generally include a two sided thrust disk rotating element, non-rotating compliant foil members that axially enclose the rotating element, non-rotating compliant underspring members that axially enclose the foil members, and a non-rotating thrust plate element and a non-rotating housing element that axially enclose and provide attachments for the foil members. The space between the rotating element and the thrust plate element on one side of the thrust disk and the space between the rotating element and the thrust surface of the housing element on the other side of the thrust disk are filled with fluid (such as air, natural gas or LPG) that envelops the foils.
The rotary motion of the rotating element applies viscous drag forces to the fluid and induces circumferential flow of the fluid between the smooth surface of the rotating element and the foil. The space between the rotating element and the compliant foil is subdivided into a plurality of fluid-dynamic wedge channels. Leading ramps of the compliant foil pads relative to the fluid""s circumferential flow and a smooth surface of the rotating element form the two primary surfaces of the converging wedge channels. Trailing edge and the smooth surface of the rotating element form the primary surfaces of the diverging wedge channels.
Fluid flowing circumferentially along a converging wedge channel experiences steadily decreasing flow area, increasing circumferential flow velocity and static fluid pressure. If the rotating element moves toward the non-rotating element, the flow area along the wedge channel decreases, causing the fluid pressure differential along the channel to increase. If the rotating element moves away, the pressure differential along the wedge channel decreases. Thus, the fluid in the wedge channels exerts restoring forces on the rotating element that vary with, and stabilize running clearances, and prevent contact between the rotating and non-rotating elements of the rotor bearing system. Flexing and sliding of the bearing foils cause coulomb damping of any axial or overturning motion of the rotating element of the rotor bearing system.
Compliant foil fluid film thrust bearings operate with extremely small running clearances. The clearances between the compliant foil""s converging channel ramp trailing ends and the rotating thrust disk are typically less than 100 micro-inches (2.5 micrometers) when the bearing is heavily loaded at operating conditions. Furthermore, the use of these thrust bearings results in moderate drag and power consumption.
Compliant foil fluid film thrust bearings tend to rely on backing or undersprings to preload the compliant foils against the rotating thrust disk so as to control foil position/nesting and to ensure rotor dynamic stability. The bearing starting torque (which should ideally be zero) is directly proportional to these preload forces and/or gravity forces. These preload forces also significantly increase the thrust disk speed at which the hydrodynamic effects in the wedge channels are strong enough to lift the rotating element of the rotor bearing system out of physical contact with the non-rotating members of the rotor bearing system. These preload forces and the high lift-off/touch-down speeds result in significant bearing wear each time the disk is started or stopped. This wear can generally be reduced significantly by coating the compliant foil members with solid film lubricants.
In accordance with a preferred embodiment of the present invention, a method for rotatably supporting a thrust disk on a thrust plate provides a compliant foil thrust bearing between the thrust disk and the thrust plate, and mounting an underspring member between the compliant foil member and the thrust plate to provide variable spring stiffness to the annular compliant foil member in both circumferential and radial directions. In another embodiment, a compliant foil fluid film thrust bearing includes a thrust disk rotatably supported by a non-rotating thrust bearing surface, and a compliant foil thrust bearing is operably disposed between the thrust disk and the non-rotating thrust bearing surface and mounted on the thrust bearing surface, the compliant foil thrust bearing includes a compliant foil member and an underspring member mounted on the thrust bearing surface and disposed between the thrust bearing surface and compliant foil member, the underspring member includes means to provide variable spring stiffness to the annular compliant foil member in both the circumferential and radial directions. Various shapes and configurations of the bearing members are illustrated and described.